Super High Threshold Percolative Ferroelectric/Ferrimagnetic Composite Ceramics with Outstanding Permittivity and Initial Permeability

Abstract

Ferroelectric/ferromagnetic composites (FFC) have attracted increasing attention because of their ferroelectric and magnetic properties. [1][2][3][4][5] They can be used to produce inductancecapacitance integrated filters, [6,7] which will reduce the space occupied on circuit boards and promote integration and miniaturization. [8] However, the capacitance (inductance) reduces with reducing the ferroelectric (ferromagnetic) phase fraction according to the compound law. Therefore, it is hard to obtain both high capacitance and high inductance from general FFCs. It is well known that the dielectric properties may be distinctly increased through the percolation effect near the percolation threshold. [8][9][10][11][12][13][14] However, the universal percolation threshold is very low (< 0.2). Even though we have prepared a percolative FFC with comparatively high percolation threshold (about 0.5) and high dielectric properties (about 10 times higher than that of the original perovskite phase) by the conventional ceramic method (CCM), [4] the percolation threshold (the content of ferrite phase) was not high enough and the permeability was only 20 % of pure magnetic ferrite. Therefore, the content of the ferrite phase should be significantly increased while the ferrite particles are prevented from connecting with each other. These requirements seems very hard to achieve by CCM. [4,[8][9][10][11][12][13][14][15][16] Therefore, it is of great importance to find a new method to produce the ferrite well surrounded by only a thin perovskite layer, which can avoid connection among the ferrite particles. If this can be achieved, on the one hand, the percolation threshold and thus the permeability of the FFCs can be increased, and, on the other hand, the permittivity can be increased significantly owing to the thinner perovskite layer. It is well known that molecular scale homogeneity can be ideally realized by sol-gel processes, and consequently homogeneous dispersions of components in composites can be obtained. In addition, the sol-gel method can be used for in situ formation of two phases, either alternately arranged or homogeneously dispersed as a two-phase system. A high threshold system with a high content and low connection of the conductive phase would likely be formed in this way.Herein, we propose two typical biphasic systems and a sol-gel preparation method, as well as an in situ multiphase formation process. The first system is PbTiO 3 (PTO)/NiFe 2 O 4 (NFO). The second is BaTiO 3 (BTO)/Ni 0.5 Zn 0.5 Fe 2 O 4 (NZFO). It is worth noting that PTO and BTO are typical ferroelectric phases with low conductivity (s % 10 À10 S cm À1 ), and NFO and NZFO are typical ferrimagnetic phases with comparatively high conductivity (s % 10 À3 S cm À1 ) and are suitable for percolating conditions.In this work, homogenously dispersed multiphase composites with extremely high content of ferrimagnetic phases were formed in situ by sol-gel processes. Typical XRD patterns of the PTO/NFO and BTO/NZFO ceramic composites (Figure 1) exhibit pea...

Formation of Sol–Gel In Situ Derived BTO/NZFO Composite Ceramics with Considerable Dielectric and Magnetic Properties

“…It demonstrates evidently that the grain size of NZFO particles increases with increasing sintering temperature and attains to 20–50 μm in diameter above 1300°C, but the grain size of the BTO particles is much smaller comparatively. By approximately measuring the average grain size from SEM images, it is estimated that the dimension of BTO particles decreases from 5 to 10 μm at f NZFO = 0.1 to 1–2 μm at f NZFO = 0.9, while the dimension of NZFO particles increases from 1 to 2 μm at f NZFO = 0.1 to 10–20 μm at f NZFO = 0.9 . As is shown, the ferrite grains exhibit a staircase growth and exist in hexagonal shape in the composite system.…”

“…Ferrite particles are good choice as magnetic filler for the preparation of ferroelectric/ferromagnetic composites, because they can bring high permeability and magnetoelectric effect to the composite system . Composite materials made from BaTiO 3 (BTO) and Ni–Zn ferrite (NZFO), the former of which has a high permittivity of about 2000 and the latter a ferrite with both high permeability (larger than 200) as well as large saturation magnetization, have been attracting researchers' interest due to the potential they show in future technologies . However, a paradox caused by the composite law imposes challenging restrictions on the comprehensive property enhancement of BTO/NZFO system, that is, the properties originated from either of the two constituent phases will generally be weakened steeply with the decrease in its content in the composite.…”

“…Percolative composite multiferroics, in which the conductivity of constituent phases varies by several orders of magnitude, are potential candidates to solve the problem. We reported the observation of percolation effect in BTO/NZFO composite system with super high threshold, and found that the percolation threshold f c is different for BTO/NZFO composite ceramics via different preparation method, i.e., f c = 0.52 for solid‐state process and f c = 0.8–0.9 for sol–gel method . In a sol–gel in situ process, the constituent can contact with each other molecularly and distribute uniformly, enabling it to form an enwrapped structure by separating the relatively conductive NZFO phase with dielectric BTO phase.…”

“…Also, homogenous distribution of constituent phases makes the properties more stable than those prepared by conventional method. Following this concept, previous researches successfully modified the dielectric and magnetic properties of BTO/NZFO composite ceramics by improving its permittivity to as high as 100 000 and initial permeability toward the value of single ferrite phase through sol–gel method and percolation effect …”

Multi-susceptibile Single-Phased Ceramics with Both Considerable Magnetic and Dielectric Properties by Selectively Doping

“…To contribute multi susceptibilities such as magnetic and dielectric properties simultaneously, percolative ceramic composites composed of ferroelectric and ferromagnetic phases have been extensively studied in recent decades123456. The percolative composites are impressive to contribute both extraordinary dielectric and magnetic performances, because the paradox caused by composite law is solved78. However, the mismatched interfaces or grain boundaries originating from the inevitable contact between dissimilar lattice structures will be introduced, thus deficiencies as well as space charges will appear in the composite ceramics.…”